Effect of Hydrogen Termination on Carbon K-Edge X-ray Absorption Spectra of Nanographene

Carbon K-edge X-ray absorption spectra of nanographene have been simulated by density functional theory calculations to obtain information on the edge termination by hydrogen. Such information is crucially important to understand and predict functions such as transport and catalysis. Our results show that different edge terminations significantly affect the binding energy of the Is core-level of C atoms in the vicinity of edges because of the change in chemical bonding and the localized edge states. We find that a shoulder or a peak appears below the pi* peak at relatively different positions with respect to the pi* peak position in the theoretical spectra of zigzag graphene nanoribbons, depending on the ratio of monohydrogen- to dihydrogen-terminations. We also point out that the two additional features observed between the pi* and sigma* peaks of an ideal graphene originate from the sigma* states of C-H bonding and C-H-2 bonding at the edges.